Printable thermoelectric materials for energy harvesting

Thermoelectric (TE) materials can harvest energy by converting waste heat into electricity. In recent years, polymers have become promising materials for near room TE applications. Novel features, such as their flexibility, lightweight, nontoxicity, low thermal conductivity, and high chemical stability, make them promising candidates for these applications. However, among the inorganic TE materials, BiTe-based materials show high TE performance at room temperature. Besides, inorganic TE materials usually have several drawbacks, such as scarcity of materials, high cost of production, toxicity, and processing difficulties. This thesis has been focused on three different TE materials: 1) conducting organic materials, 2) insulating organic materials, and 3) ionic organic materials. In the first part of the thesis, the TE properties of Poly (3-hexylthiophene) (P3HT) is investigated. P3HT is a conducting conjugated polymer that has been thoroughly examined in various fields, including organic TE materials. However, pristine P3HT has a low electrical conductivity, and it needs to be increased for many applications. We investigated the relationship between the TE parameters and the molecular weight (MW) of P3HT thin films. In addition, the electrical properties of P3HT were improved by LiTFSI and TBP as additives. We showed the relevant effects of MW on the structure of polymer films, such as morphological compactness, distribution of additives, and the degree of interchain order. To further improve the TE properties of P3HT, the same formulations are blended with carbon nanotube forest (CNTF). CNTF together with additives significantly improved the TE properties of P3HT. In the second section, the printable TE materials have been developed with Ethyl cellulose (EC), an insulator polymer, and Graphene nanoplatelets (GNPs). The weight ratio of GNPs and EC were ranged from 0.2 to 0.7. These conductive pastes have been deposited by blade coating on glass substrates. The electrical conductivity of the composites has increased polynomially as the filler content increased, whereas the Seebeck coefficient did not change significantly with the increased electrical conductivity. Moreover, a 3D structure form (cylindrical pellet) from the highest conductive paste was also fabricated. In the last part, we investigated the ionic TE materials. In these materials, instead of electrical charges, the ions thermodiffuse under a temperature gradient through the Soret effect. When the temperature difference was applied, the ions cannot pass into an external circuit and they accumulated at the electrode/electrolyte interface to form an electric double layer. So, they convert thermal energy into stored electrical energy. This principle can be used to charge a supercapacitor or a battery. Here, we investigated the thermocapacitive properties of PEDOT:PSS treated with DMSO in a non-aqueous polymer electrolyte gel. This is a new concept to harvest energy, particularly suitable for intermittent heat sources like the sun.